1. Field
The present invention relates to treads for tires fitted to heavy goods vehicles and more particularly relates to the tread patterns of these treads.
2. Description of Related Art
Driving in the rain requires the water in the region of contact between each tire and the roadway to be removed as swiftly as possible in order to ensure contact between the material of which the tread is made and this roadway. To achieve this, grooves are formed on the tread, which grooves, on account of their dimensions (depth and width) remain open in the contact patch for contact with the roadway and thus allow water which is not pushed forward and around the sides of the tire to flow away.
The grooves may have any shape when viewed in cross section and in terms of the lines that they follow along the tread surface and may be oriented in any direction. What is meant by the line followed by a groove along the tread surface is the mean geometric line followed by the corner edges that the said groove forms on the said surface.
In the case of the tires intended for the steered or load-bearing axles of a heavy goods vehicle, it is common practice for the tread of these tires to be provided with longitudinal grooves the depth of which is equal to or substantially equal to the total thickness of the tread (not including any thickness of tread which may have been provided with a view to allowing the grooves to be partially renewed through a regrooving operation). As a general rule, this depth is between 13 and 18 mm on these axles. For the tires intended for the driven axles, the groove depth is greater and may be as much as 24 mm. The “total thickness of a tread” here means the total thickness of material that can be worn away before a tire has to be changed or reconditioned by retreading.
In the case of such tires of the prior art, the total void ratio is, as a general rule, between 15 and 25% of the volume of tread intended to be worn away during driving. It is found that these tires have an available void volume in the contact patch which is relatively high in the new state; this void volume opening onto the tread surface in the contact patch is, on average, of the order of 100 cm3 for example for a tire of size 315/70 R 22.5. For the tire in question, this value is obtained for its nominal inflation pressure and static loading conditions as set down in particular in the ETRTO standard.
Moreover, in order to improve the grip between the tire and the roadway it is known practice to form edge corners of rubber on the tread surface. To form such edge corners, sipes are made, these being thin incisions of an average width that is such that, under normal loading conditions, the walls of material delimiting each sipe can, at least partially, come into contact with one another as they pass through the contact patch in which the tire makes contact with the road surface so as to limit the loss of rigidity associated with the presence of the sipes. These sipes may be of a depth equal to the thickness of the tread to be worn away or less than this thickness.
While grooves, or more generally, cavities, are essential to draining water away, the resulting reduction in surface area of material may have an appreciable effect on the wear rate performance of a tread and therefore reduce the service life of the tire as the result of an increase in the wear rate. Other tire performance aspects may also be affected, notably its behavior, road noise and rolling resistance. It is also found that these grooves which are formed to have a working depth equal to the height of tread to be worn away may be the cause of endurance problems. Under certain driving conditions, foreign objects such as stones may become lodged in these grooves and attack the bottoms of these grooves and cause breakages to appear in the rubber. It has also been observed that manoeuvres involving mounting and descending from various obstacles can cause breakages at the bottoms of certain grooves, notably those situated near the lateral edges of the tread.
It is known that the presence of grooves in a tread generates a reduction in the compression and shear stiffness of this tread because these grooves delimit portions of material which deform more readily by comparison with the portions delimited by sipes the walls of which can come into contact with one another. This reduction in stiffness, in the case of the presence of grooves, leads to an increase in deformation and generates a reduction in the wear performance of the tread: greater wear for a set distance covered is observed (which corresponds to an increase in the tread wear rate). Moreover, an increase in the rolling resistance and therefore in the fuel consumption of vehicles equipped with such tires is noted, this being the result of an increase in hysteresis losses which are associated with the deformation cycles of the material of which the tread is made.
Bearing in mind the thickness that their tread needs to have, the usual tires used on heavy goods vehicles have, in the region of the edges of the tread, operating temperatures which govern the choice of rubber materials used in the tread and notably require the use of compositions the hysteresis of which is limited.
Document WO 03/097384 A1 discloses a tire for a heavy goods vehicle having a tread comprising grooves that are open to the tread surface and underlying cavities beneath the tread surface in the new state, each of these cavities having a top generatrix that follows a wavy profile.